In the etching of metal webs, and, in particular, in the etching of metal webs from opposite sides it is difficult to uniformly control the distribution of etchant throughout the metal web. If holes are being etched in the metal web, the size and shape of the holes may vary substantially as a result of non-uniform etchant distribution.
One method used to more uniformly distribute the etchant is to place a first set of oscillateable etchant spray nozzles above the metal web and a second set of oscillateable etchant spray nozzles below the metal web, with both sets of oscillateable nozzles spraying etchant directly onto the metal web. The result is an etching process which has better dimensional controls, since the oscillating nozzles can more uniformly distribute the etchant on the metal web.
However, even with oscillating the nozzles, the spray patterns are not uniform, often resulting in uneven etching. The problem with uneven etching is that once breakthrough occurs in a metal web, that is, a hole has been formed in the web, etching proceeds at a much more rapid rate, since fresh etchant is continuously applied to the sides of the hole. The result is that a first pre-breakthrough etching rate exists, and a second post-breakthrough etching rate exists after the opening or hole is formed. Consequently, if all the holes in the metal are not begun to be enlarged at the same time the holes can become irregular and misshapen as a result of the different etching rates before and after breakthrough.
One of the goals of the present invention is to more uniformly distribute etchant to have the breakthrough occur at substantially the same time throughout the metal web. If the rate of etching proceeds at a constant rate throughout the metal web, one can accurately control the final dimensions of any openings formed in the metal web.
The present invention provides a process and apparatus for more uniformly distributing the etchant across the metal web in order to obtain breakthrough at substantially the same time throughout the metal web.